Artificial intervertebral disc with lower height

ABSTRACT

An intervertebral disc is provided having an upper plate, a lower plate, and a core. The upper and lower plates include outer vertebral body contacting surfaces which are provided with attachment enhancing features to ensure bone integration. The attachment enhancing features shown include one or more fins, serrations and teeth. An opposite surface of the plates from the vertebral body contacting surfaces is formed with a recess which serves as a bearing surface for the core. In order to form an intervertebral disc with a lower disc height, at least one of the recesses is provided opposite a corresponding dome shaped portion on the vertebral body contacting surfaces. This allows the plates to be formed with a thinner profile for a smaller overall disc height. In addition to providing a lower overall height to the artificial disc, the dome shaped portion of the plates also provides a more anatomically shaped outer vertebral body contacting surface.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/069,048, filed Mar. 11, 2008, entitled “ARTIFICIAL INTERVERTEBRALDISC WITH LOWER HEIGHT” the full disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to medical devices and methods. Morespecifically, the present invention relates to intervertebral discprostheses.

Back pain takes an enormous toll on the health and productivity ofpeople around the world. According to the American Academy of OrthopedicSurgeons, approximately 80 percent of Americans will experience backpain at some time in their life. On any one day, it is estimated that 5%of the working population in America is disabled by back pain.

One common cause of back pain is injury, degeneration and/or dysfunctionof one or more intervertebral discs. Intervertebral discs are the softtissue structures located between each of the thirty-three vertebralbones that make up the vertebral (spinal) column. Essentially, the discsallow the vertebrae to move relative to one another. The vertebralcolumn and discs are vital anatomical structures, in that they form acentral axis that supports the head and torso, allow for movement of theback, and protect the spinal cord, which passes through the vertebrae inproximity to the discs.

Discs often become damaged due to wear and tear or acute injury. Forexample, discs may bulge (herniate), tear, rupture, degenerate or thelike. A bulging disc may press against the spinal cord or a nerveexiting the spinal cord, causing “radicular” pain (pain in one or moreextremities caused by impingement of a nerve root). Degeneration orother damage to a disc may cause a loss of “disc height,” meaning thatthe natural space between two vertebrae decreases. Decreased disc heightmay cause a disc to bulge, facet loads to increase, two vertebrae to rubtogether in an unnatural way and/or increased pressure on certain partsof the vertebrae and/or nerve roots, thus causing pain. In general,chronic and acute damage to intervertebral discs is a common source ofback related pain and loss of mobility.

When one or more damaged intervertebral discs cause a patient pain anddiscomfort, surgery is often required. Traditionally, surgicalprocedures for treating intervertebral discs have involved discectomy(partial or total removal of a disc), with or without fusion of the twovertebrae adjacent to the disc. Fusion of the two vertebrae is achievedby inserting bone graft material between the two vertebrae such that thetwo vertebrae and the graft material grow together. Oftentimes, pins,rods, screws, cages and/or the like are inserted between the vertebraeto act as support structures to hold the vertebrae and graft material inplace while they permanently fuse together. Although fusion often treatsthe back pain, it reduces the patient's ability to move, because theback cannot bend or twist at the fused area. In addition, fusionincreases stresses at adjacent levels of the spine, potentiallyaccelerating degeneration of these discs.

In an attempt to treat disc related pain without fusion, an alternativeapproach has been developed, in which a movable, implantable, artificialintervertebral disc (or “disc prosthesis”) is inserted between twovertebrae. A number of different artificial intervertebral discs arecurrently being developed. For example, U.S. Patent ApplicationPublication Nos. 2005-0021146, 2005-0021145, and 2006-0025862, which arehereby incorporated by reference in their entirety, describe artificialintervertebral discs. This type of intervertebral disc has upper andlower plates positioned against the vertebrae and a mobile corepositioned between two plates to allow articulating, lateral androtational motion between the vertebrae.

Another example of an intervertebral disc prostheses having a movablecore is the CHARITE artificial disc (provided by DePuy Spine, Inc.) anddescribed in U.S. Pat. No. 5,401,269. Other examples of intervertebraldisc prostheses include MOBIDISC (provided by LDR Medical), the BRYANCervical Disc (provided by Medtronic Sofamor Danek, Inc.), and thePRODISC (from Synthes Stratec, Inc.) and described in U.S. Pat. No.6,936,071. Some of these intervertebral discs are mobile core discswhile others have a ball and socket type two piece design. Althoughexisting disc prostheses provide advantages over traditional treatmentmethods, improvements are ongoing.

These known artificial intervertebral discs generally include upper andlower plates which locate against and engage the adjacent vertebralbodies, and a core for providing motion between the plates. The core maybe movable or fixed, metallic, ceramic or polymer and generally has atleast one convex outer surface which mates with a concave recess on oneof the plates in a fixed core device or both of the plates for a movablecore device. In order to implant these intervertebral discs, the naturaldisc is removed and the vertebrae are distracted or forced apart inorder to fit the artificial disc in place. Depending on the size of thedisc space, many of the known artificial discs have a height which ishigher than desired resulting in an unnaturally over distractedcondition upon implantation of the disc. For example, a smallest heightdisc available can be about 10-13 mm for lumbar discs and about 5-6 mmfor cervical discs.

Currently available artificial intervertebral discs do not provide adesired low profile for some patients with smaller disc heights. The useof an artificial disc which is too large for a patient's disc space canresult in limited mobility of the disc. An improperly sized artificialdisc can also move out of place in the disc space. Accordingly, it wouldbe desirable to provide a lower height disc which mimics more closelythe natural anatomy for smaller patients.

In addition, the vertebral body contacting surfaces of many of the knownartificial discs are flat. This flat configuration does not generallymatch the surfaces of the vertebral bodies resulting in less than idealbone to implant contact surfaces. It would be desirable to provide amore anatomically shaped vertebral body contacting surface for anartificial disc.

Therefore, a need exists for an improved artificial intervertebral discwith a lower disc height and more anatomically shaped upper surface.

BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided an intervertebralprosthesis for insertion between adjacent vertebrae, the prosthesiscomprising upper and lower prosthesis plates with domed outer surfaceslocatable against adjacent vertebral bodies.

In accordance with another aspect of the invention, an intervertebraldisc includes an upper plate having an upper vertebra contacting surfaceand a lower bearing surface and a lower plate having a lower vertebracontacting surface and an upper surface. The upper surface of the upperplate has a convex central portion and at least one anchoring elementextending from an upper surface of the convex central portion. The lowerbearing surface of the upper plate has a concavity disposed opposite theconvex central portion. The lower surface of the lower plate has aconvex central portion and the upper bearing surface of the lower platehas a concavity disposed opposite the convex central portion. A core ispositioned between the upper and lower plates, the core having upper andlower convex surfaces configured to mate with the bearing surfaces ofthe upper and lower plates.

In accordance with a further aspect of the invention, an intervertebraldisc includes a first plate having a first vertebrae contacting surfaceand a bearing surface opposite the first vertebrae contacting surface,and a second plate having a second vertebrae contacting surface and anopposite surface and wherein the second plate articulates with respectto the first plate. The first vertebrae contacting surface of the firstplate has a convex central dome shaped portion and at least oneanchoring fin extending from an outer surface of the convex central domeshaped portion, and the bearing surface of the first plate has aconcavity directly opposed to the convex central portion.

In accordance with an additional aspect of the invention, anintervertebral disc includes an upper plate having an upper vertebracontacting surface and a lower surface, and a lower plate having a lowervertebra contacting surface and an upper surface, and the upper platearticulates with respect to the lower plate. The upper surface of theupper plate has at least one anchoring fin extending from the uppersurface of the upper plate and the lower surface of the lower plate hasat least one anchoring fin extending from the lower surface of the lowerplate. A plurality of teeth are positioned in a symmetrical pattern onopposite sides of the anchoring fins of the upper and lower plates, theplurality of teeth each having a first surface and a second surfaceopposite the first surface, wherein the first surface is formed at anangle of less than 40 degrees with respect to the vertebra contactingsurfaces and the second surface is formed at an angle of more than 50degrees with respect to the vertebra contacting surfaces. A plurality ofserrations surround the plurality of teeth, wherein the plurality ofserrations have a height at least 25 percent lower than a height of theteeth.

Other features of the invention are set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an intervertebral disc according toone embodiment of the present invention;

FIG. 2 is a top view of the intervertebral disc of FIG. 1;

FIG. 2A is a side profile view of the teeth shown in FIG. 1;

FIG. 3 is a perspective view of the intervertebral disc of FIG. 1; and

FIG. 4 is a perspective view of an intervertebral disc according toanother embodiment of the invention.

FIG. 5 is a perspective view of an intervertebral disc plate accordingto another embodiment of the invention with a bearing surface insert.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-3 illustrate an intervertebral disc having an upper plate 10, alower plate 12, and a core 14. The upper and lower plates 10, 12 includeouter vertebral body contacting surfaces which are provided withattachment enhancing features to ensure bone integration. The attachmentenhancing features shown include one or more fins 16, serrations 18 andteeth 20. The fin 16 can be an elongate fin pierced by one or moretransverse holes 22. The disc can be inserted posteriorly into thepatient from an anterior access, such that fin 16 can enter a groove inone of the vertebrae as a posterior side P of the intervertebral discenters the intervertebral space followed by an anterior side A of theintervertebral disc. An opposite surface of the plates from thevertebral body contacting surfaces is formed with a recess 24 whichserves as a bearing surface for the core 14. In order to form anintervertebral disc with a lower disc height, at least one of therecesses 24 is provided opposite a corresponding dome shaped portion 26on the vertebral body contacting surfaces. This allows the plates 10, 12to be formed with a thinner profile for a smaller overall disc height.

In addition to providing a lower overall height to the artificial disc,the dome shaped portion 26 of the plates also provides a moreanatomically shaped outer vertebral body contacting surface. Thesurfaces of the vertebral bodies are commonly not flat, but instead havea slight dish shape to their center portion. The dome shaped portion 26fits well into this dish shape of the vertebrae providing better bonecontact and improved bone integration.

The core 14 can be formed as a circular disc shaped member with upperand lower bearing surfaces 28 which match the curvature of the recesses24. The core 14 also has an annular rim 30 which cooperates with aretention feature 32 on at least one of the discs to retain the corebetween the plates when the intervertebral disc is implanted between thevertebrae of a patient. The core 14 is moveable with respect to both theupper and lower discs to allow articulation, translation and rotation ofthe upper and lower plates with respect to one another. The coresurfaces 28 and recess surface 24 have the same radius of curvaturewhich may vary depending on the size of the intervertebral disc.

In the embodiment shown in FIGS. 1-3 a single central fin 16 is providedon each of the plates 10, 12 extending in an anterior posteriordirection with an angled posterior edge for aiding in insertion. Thisembodiment is particularly useful for insertion from an anterior side ofthe intervertebral disc space. Alternatively, two or more fins 16 canalso be provided on each plate. In one example, a single fin can beprovided on one plate while double fins can be symmetrically arranged onthe other plate to achieve a staggered arrangement particularly usefulfor multi-level disc implant procedures. This staggered arrangementprevents the rare occurrence of vertebral body splitting by avoidingcuts to the vertebral body in the same plane for multi-level implants.The orientation of the fin(s) 16 can also be modified depending on theinsertion direction for the intervertebral disc 10. In alternativeembodiments, the fins 16 may be rotated away from the anterior-posterioraxis, such as in a lateral-lateral orientation, aposterolateral-anterolateral orientation, or the like for implantationin the associated directions.

The fins 16 are configured to be placed in slots in the vertebralbodies. In one embodiment, the fins 16 are pierced by transverse holes54 for bone ingrowth. The transverse holes 54 may be formed in any shapeand may extend partially or all the way through the fins 50, 52.Preferably, the fins 16 each have a height greater than a width and havea length greater than the height.

The fins 16 provide improved attachment to the bone and prevent rotationof the plates in the bone. In some embodiments, the fins 16 may extendfrom the surface of the plates 10, 12 at an angle other than 90°. Forexample on one or more of the plates 10, 12 where multiple fins 16 areattached to the surface the fins may be canted away from one anotherwith the bases slightly closer together than their edges at an anglesuch as about 80-88 degrees. The fins 16 may have any other suitableconfiguration including various numbers angles and curvatures, invarious embodiments. In some embodiments, the fins 16 may be omittedaltogether.

The teeth 20 shown in FIG. 1 to FIG. 3 include six teeth arranged in achevron shape with the narrow portion of the chevron located on aposterior side of the plates 10, 12. However, other numbers of teeth andteeth arrangements may also be used. For example, the teeth 20 may bearranged, as shown in FIG. 4, in an alternating arrangement such thatthe first and third teeth on each side are aligned and located closer tothe midline of the disc while the central tooth is located further fromthe midline (a diamond shape pattern). The teeth can be positioned invarying numbers and arrangements depending on the size and shape of theplate used.

The teeth 20 as shown in FIG. 1 to FIG. 3 have an elongated shape withopposite triangular side surfaces. As shown most clearly in FIG. 2A,each of teeth 20 comprises a posterior surface 20P and anterior surface20A. In the embodiment of FIGS. 1-3, the teeth each have posteriorsurface 20P opposite anterior surface 20A, wherein the posterior surfaceis formed at an angle 21P of less than 40 degrees with respect to thevertebra contacting surfaces of the plates 10, 12, and the anteriorsurface 20A is formed at an angle 21A of more than 50 degrees withrespect to the vertebra contacting surfaces. Preferably, the posteriorsurface 21P is formed at an angle of about 5 to about 30 degrees, whilethe anterior surface 21A is formed at an angle of about 70 to about 90degrees. In addition, the anterior surface 21A can be canted in theanterior direction to provide a tooth in the form of a barb. In thisway, the posterior surface 21P provides minimal resistance to insertionwhile the anterior surface 21A prevents backing out of the disc.

Although wedge shaped teeth are shown, other teeth shapes may also beused, for example pyramidal, conical, rectangular and/or cylindricalteeth.

Referring now to FIG. 4, vertebrae contacting surfaces may comprisefinlets 40. Finlets 40 can be somewhat larger than teeth and differ fromteeth in that they have an additional top surface. The finlets 40 mayhave a tapered top surface 42 and larger base portion 44. The teeth 20and/or finlets 40 may have a height which is preferably greater thanthat of the serrations. The teeth and/or finlet heights can be about0.5-3 mm, preferably about 1-2 mm and more preferably about 1.1-1.5 mm.In the example of FIG. 4, the finlets 40 have an anterior surface formedat an angle of about 70 to 90 degrees, preferably about 80 degrees aposterior surface formed at an angle of about 50-70 degrees, preferablyabout 60 degrees, and a top surface formed at an angle of about 2-20degrees, each with respect to the vertebra contacting surfaces.

The dome shaped portion(s) 26 on the vertebral body contacting surfacesare provided to allow the core 14 to be recessed further into the upperand lower plates 10, 12. The dome shaped portion preferably has adiameter less than a diameter of the core, i.e. less than about 20 mmfor a lumbar disc and less than about 10 mm for a cervical disc. In onepreferred embodiment, the external dome 26 has a diameter of about 10-20mm for lumbar and about 5-10 mm for cervical. The dome in one embodimenthas a height of about 0.5-3 mm above a plane formed by the vertebralbody contacting surfaces of the plates at the base of the serrations,when serrations are present. Preferably, the dome height is about 0.7-2mm.

In the embodiment shown in FIGS. 1-3, some of the serrations 18 arelocated on the surface of the dome shaped portion 26, with the tops ofthe serrations even with the serrations which are not positioned on thedome. Alternatively, the serrations 18 on the dome 26 can extendsomewhat higher than the other serrations on the plates or serrationscan be omitted on the dome shaped portion. The serrations 18 as shownare pyramid shaped serrations extending in mutually orthogonaldirections, however, other shapes may also be used. The serrations havea height of about 0.5-1 mm and a width about equal to their height. Withpassage of time, firm connection between the plates 10, 12 and thevertebrae will be achieved as bone tissue grows over the serratedfinish. Bone tissue growth will also take place about the fins 16 andthrough the holes 22 therein, further enhancing the connection which isachieved.

Other geometries of bone integration structures may also be usedincluding teeth, grooves, ridges, pins, barbs or the like. When the boneintegration structures are ridges, teeth, barbs or similar structures,they may be angled to ease insertion and prevent migration. These boneintegration structures can be used to precisely cut the bone duringimplantation to cause bleeding bone and encourage bone integration.Additionally, the outer surfaces of the plates 10, 12 may be providedwith a rough microfinish formed by blasting with aluminum oxidemicroparticles or the like to improve bone integration. In someembodiments, the outer surface may also be titanium plasma sprayed or HAcoated to further enhance attachment of the outer surface to vertebralbone.

The core 14 according to the embodiments of FIGS. 1-4 can be retained inthe lower plate 12 by retention feature 28 comprising a retention ringthat protrudes inwardly from an edge of the lower plate 12 at an edge ofthe lower bearing surface 32 of the plate. Although a circumferentialcore retaining feature is shown, other core retaining features may alsobe used including at least those shown in U.S. Patent Publication Nos.2005/0251262, 2005/0021146, and 2005/0021145, which are incorporatedherein by reference in their entirety.

Although the core 14 has been shown as circular in cross section withspherically shaped bearing surfaces 28, other bearing surface shapes maybe used including oval, elliptical, or kidney bean shaped. Thesenon-circular shaped cores can be used to limit rotational motion betweenthe upper and lower plates 10, 12. The bearing surface shapes may alsovary from the spherical shapes shown and may vary between the upper andlower surfaces of the core. Other bearing surface shapes includecylindrical, elliptical, trough shaped, groves, flat surfaces, or thelike. Although the core 14 and plates 10, 12 have been shown as solidmembers, the core and plates may be made in multiple parts and/or ofmultiple materials. The core can be made of low friction materials, suchas titanium, titanium nitrides, other titanium based alloys, tantalum,nickel titanium alloys, stainless steel, cobalt chrome alloys, ceramics,or biologically compatible polymer materials including PEEK, UHMWPE, PLAor fiber reinforced polymers. High friction coating materials can alsobe used.

The intervertebral disc according to the present invention providesarticulation in two directions as well as rotation. The plates 10, 12are provided with grooves 34 at their lateral edges for use in graspingthe disc by an instrument to facilitate holding and manipulation of thedisc for insertion or removal of the disc. The grooves 34 allow theplates 10, 12 to be grasped and inserted simultaneously in a lockedorientation.

The upper and lower plates 10, 12 may be formed from titanium, titaniumnitrides, other titanium based alloys, tantalum, nickel titanium alloys,stainless steel, cobalt chrome alloys, ceramics, or biologicallycompatible polymer materials including PEEK, UHMWPE, PLA or fiberreinforced polymers. The bearing surfaces or recesses 24 are concavely,spherically curved and can have a hard coating such as a titaniumnitride finish. The plates 10, 12 may be treated with aluminum oxideblasting followed by a titanium plasma spray to improve boneintegration. Other materials and coatings can also be used such astitanium coated with titanium nitride, aluminum oxide blasting, HA(hydroxylapatite) coating, micro HA coating, and/or bone integrationpromoting coatings. Any other suitable metals or combinations of metalsmay be used as well as ceramic or polymer materials, and combinationsthereof to optimize imaging characteristics. Any suitable technique maybe used to couple materials together, such as snap fitting, slipfitting, lamination, interference fitting, use of adhesives, weldingand/or the like.

FIG. 5 illustrates an alternative embodiment of an intervertebral discplate 100 having a metallic vertebral body contacting portion 102 and abearing surface insert 104 formed of PEEK or other MRI compatiblematerial. The PEEK insert 104 includes the bearing surface (not shown)and has a dome shaped central portion. The metallic portion 102 includesa central circular opening 106 which can be of any shape andaccommodates a portion of the PEEK insert 104. The metallic portion 102may be formed in two parts and welded around the PEEK insert 104 to forma firm connection between the metallic and PEEK portions of the plate.The PEEK insert provides the advantage of MRI compatibility while themetallic vertebral body contacting portion 102 provides a superiorsurface for bone integration and includes one or more fins 110, finlets112, and serrations 114 as described with respect to the otherembodiments described herein.

Each of the artificial intervertebral discs as described herein issurgically implanted between adjacent spinal vertebrae in place of adamaged disc. Those skilled in the art will understand that the surgicaltechnique for implanting the artificial discs involves partial or totalremoval of the damaged disc and distraction of the adjacent vertebrae byforcibly separating the vertebrae from one another to provide thenecessary space for insertion of the disc. The plates 10, 12 are slippedinto place between the vertebrae with their fins 16 entering slots cutin the opposing vertebral surfaces to receive them. The plates may beinserted simultaneously with or without the core. After partialinsertion of the disc, the individual plates 10, 12 can be furtheradvanced independently or together to a final position. Once the dischas been inserted, the vertebra move together to hold the assembled discin place.

The vertebral contacting surfaces of the plates 10, 12 including theteeth 20 and the serrations 18 locate against the opposing vertebraeand, with passage of time, firm connection between the plates and thevertebrae will be achieved as bone tissue grows over the serratedfinish. Bone tissue growth will also take place about the fins 16 andthrough the holes 22 therein, further enhancing the connection which isachieved.

While the exemplary embodiments have been described in some detail, byway of example and for clarity of understanding, those of skill in theart will recognize that a variety of modifications, adaptations, andchanges may be employed. Hence, the scope of the present inventionshould be limited solely by the appended claims.

What is claimed is:
 1. An intervertebral disc comprising: an upper platehaving an upper vertebra contacting surface and a lower bearing surface,wherein the upper vertebra contacting surface of the upper platecomprises a vertebral contacting surface formed in a first plane, aconvex central portion extending upward from the first plane and atleast one anchoring element extending upward beyond an upper surface ofthe convex central portion, and wherein the lower bearing surface has aconcavity disposed opposite the convex central portion; a lower platehaving a lower vertebra contacting surface and an upper surface; andwherein the lower vertebra contacting surface of the lower platecomprises a vertebral contacting surface formed in a second plane and aconvex central portion extending downward from the second plane, andwherein the upper bearing surface has a concavity disposed opposite theconvex central portion; a core positioned between the upper and lowerplates, the core having upper and lower convex surfaces configured tomate with the bearing surfaces of the upper and lower plates; and aplurality of teeth provided on the upper surface of the upper plate andthe lower surface of the lower plate, wherein the teeth having aposterior facing surface formed at an angle of less than 40 degrees withrespect to the vertebra contacting surfaces and an anterior facingsurface formed at an angle of more than 50 degrees with respect to thevertebra contacting surfaces.
 2. The disc of claim 1, wherein the coreis movable with respect to the upper and lower plates.
 3. The disc ofclaim 1, wherein the at least one anchoring element is a first finhaving a height greater than a width.
 4. The disc of claim 3, whereinthe first fin has a length greater than the height.
 5. The disc of claim3, further comprising a second fin on the lower plate extending from thelower surface of the convex central portion.
 6. The disc of claim 1,wherein the plurality of teeth are not positioned on the convex centralportions of the upper and lower plates.
 7. The disc of claim 6, whereinthe teeth are positioned in a symmetrical pattern adjacent to the convexcentral portions of the upper and lower plates.
 8. The disc of claim 1,wherein the convex central portions are dome shaped portions.
 9. Thedisc of claim 8, wherein the dome shaped portions have a diameter ofabout 5 to about 20 mm.
 10. The disc of claim 1, comprising a pluralityof finlets provided on the upper surface of the upper plate and thelower surface of the lower plate.
 11. An intervertebral disc comprising:an upper plate having an upper vertebra contacting surface and a lowersurface, wherein the upper surface of the upper plate has at least oneanchoring fin extending from the upper surface of the upper plate; alower plate having a lower vertebra contacting surface and an uppersurface; and, wherein the lower surface of the lower plate has at leastone anchoring fin extending from the lower surface of the lower plate; aplurality of teeth positioned in a symmetrical pattern on opposite sidesof the anchoring fins of the upper and lower plates, the plurality ofteeth each having a first surface and a second surface opposite thefirst side surface, wherein the first surface is formed at an angle ofless than 40 degrees with respect to the vertebra contacting surfacesand the second surface is formed at an angle of more than 50 degreeswith respect to the vertebra contacting surfaces; a plurality ofserrations surrounding the plurality of teeth, wherein the plurality ofserrations have a height at least 25 percent lower than a height of theteeth; and wherein the upper plate articulates with respect to the lowerplate.
 12. The disc of claim 11, wherein the anchoring fins have alength greater than their heights.
 13. The disc of claim 11, furthercomprising a core positioned between the upper and lower plates, whereinthe core is movable with respect to the upper and lower plates.
 14. Thedisc of claim 11, wherein the at least one fin on one of the upper andlower plates comprises two parallel fins.